The overall aim of this application is to investigate mechanisms of alpha-synuclein (?syn)-induced mitochondrial dysfunction in Lewy body diseases (LBD). Despite its predominant localization in the cytosol, ?syn localizes to mitochondria in post-mortem LBD brains. Within the mitochondria, ?syn accumulation can impair complex I and IV function, decrease membrane potential, increase levels of reactive oxygen species, and increase apoptosis associated with cytochrome c release from the mitochondria. Together these data suggest an increase in mitochondrial ?syn expression and/or abnormal accumulation of toxic aggregates interferes with mitochondrial function. Maintaining mitochondrial health is essential to prevent neuronal cell death in the brain. Sirtuin 3 (SIRT3) is a NAD+-dependent protein deacetylase exclusively localized to the mitochondria where it regulates mitochondrial processes including protein deacetylation, organelle biogenesis, and oxidative stress. SIRT3 is expressed at high levels in the brain and other nervous system tissues, and can act as a pro- survival factor, playing an essential role in protecting neurons under conditions of excitotoxicity and rescuing neuronal loss in neurodegenerative models. Experimental evidence indicates that SIRT3- induced neuroprotection against oxidative stress is partially mediated by enhancement of mitochondrial biogenesis and integrity. As we consider sirtuin-based drug therapies for diseases of ageing, it is important to determine if modulating SIRT3 can protect against neurodegeneration where mitochondrial dysfunction has been demonstrated to play a role. This proposal will investigate how mitochondrial SIRT3 contributes to ?syn-induced mitochondrial dysfunction in 3 coordinated aims. In aim 1 we will perform comprehensive mitochondrial function analyses to reveal how ?syn accumulation leads to mitochondrial damage and the role of SIRT3 therein using patient-derived cells and postmortem brain. In aim 2 we will interrogate the SIRT3 regulated acetylome to identify novel targets of ?syn-associated mitochondrial dysfunction, and in aim 3 we will validate SIRT3 as a novel target for therapeutic intervention in PD in a comprehensive in vivo approach using genetic overexpression and pharmacological activation. The proposed rigorous analysis of various mitochondrial aspects will dissect causes from consequences and reveal the cross-talk between ?syn, SIRT3, and mitochondrial signaling pathways as well as oxidative and protein stress responses.